1. Field of Invention
This invention relates to an apparatus for measuring moisture, for example, moisture of paper in a paper making machine.
2. Description of the Prior Art
FIGS. 1-3 depict conventional moisture meters for measuring moisture of paper, for example, in a paper making machine. In FIG. 1, a light emitting portion 1 and a light detecting portion 2 are disposed facing each other with paper 3, whose moisture is to be measured, disposed therebetween. At light emitting portion 1, rays from a light source 6 are made to be parallel rays by a lens 7, are further made to be intermittent rays by a chopper wheel 8, and are applied to paper 3 through an irradiation window 4. Chopper wheel 8 is provided with a filter 9 which transmits rays, the wavelength of which is 1.94 .mu.m, and which are absorbed by moisture (designated as M rays) and a filter 10 which transmits rays, the wavelength of which is 1.8 .mu.m, and which are not absorbed by moisture (designated as R rays). According to the rotation of chopper wheel 8, M rays and R rays are alternately applied to paper 3. At light detection portion 2, the rays which penetrate paper 3 are introduced through an incidence window 5, focused by a lens 11, and focused on light detector 12. At light detector 12, the M rays and the R rays are detected in time sequence and supplied to computing unit 13. Then, computing unit 13 computes V.sub.R /V.sub.M and outputs the results.
In the conventional moisture meter of FIG. 2, at light emitting portion 1, rays from light source 6 are made to be parallel by lens 7, are made to be intermittent rays by a chopper wheel 8', and are applied to paper 3 through irradiation window 4. Filters, such as mounted on the meter of FIG. 1, are not mounted on chopper wheel 8', and chopper wheel 8' is used solely for the purpose of eliminating the influence of stray light. White light which is applied from irradiation window 4 is multiply reflected at irregular reflection surfaces 16,17 which are provided on the surface of light emitting portion 1 and on the surface of the light detecting portion 2, respectively. Surfaces 16,17 face each other with paper 3 sandwiched therebetween. Then, white light is introduced in light detecting portion 2 from incidence window 5 which is provided in misalignment with respect to irradiation window 4.
At light detecting portion 2, the introduced light is divided into two by a beam splitter 18. One group of divided light is introduced into light detector 12 through filter 9 which transmits the M rays and through lens 11. The other group of divided light is introduced into light detector 12' through filter 10 which transmits the R rays and through a lens 11'. The R rays which are detected by light detector 12 and the R rays which are detected by light detector 12' are supplied to computing unit 13 at the same time. Then, computing of V.sub.R /V.sub.M is conducted and the results are outputted.
FIG. 3 shows another conventional moisture meter, wherein spherical mirrors 20,21 are disposed with the openings covered by dust-proof glass 22,23 and with paper 3 sandwiched therebetween. In this meter, rays which are supplied by light source 6 and which are made to be intermittent rays by chopper wheel 8 having the two kinds of filters that are discussed above, are applied to paper 3 through an irradiation window 5. Then, the rays which penetrate paper 3 or which are scattered by paper 3 reach light detector 12 after penetrating paper 3 a plurality of times by being reflected at the inner surface of the sphere and by being applied to paper 3. The computing of V.sub.R /V.sub.M is conducted in the same manner as in FIGS. 1 and 2 by a computing unit, not shown, using the detected rays and an electrical signal which is related to the moisture of paper 3 is then outputted.
In the conventional moisture meters referred to hereinabove, M rays and R rays are applied to the paper, and the ratio of the output V.sub.R of the R rays which penetrate the paper to the output V.sub.M of the M rays which penetrate the paper (i.e. V.sub.R /V.sub.M) is computed and an electrical signal which is related to the moisture weight of the paper is obtained.
In an on-line measurement, since the relationship between the moisture weight and the output of the moisture meter subtly changes depending on the type of pulp from which the paper is made and on the basis weight of the paper, calibration curves are made using samples which are prepared in advance. Then, the calibration curves are inputted to a computer and the calibration curve having characteristics which are the nearest to the type of pulp and basis weight of the paper to be manufactured is selected, and then the moisture weight of the paper is obtained using that calibration curve.
However, the number of calibration curves which can be inputted to the computer is limited, for example, eight, and all objects to be measured can not be covered In the conventional moisture meter, even with respect to paper made from the same type of pulp, if the basis weight is different, the gap between the different calibration curves is large.
FIG. 4 shows calibration curves which are obtained using three different types of samples in the meter of FIG. 2. In FIG. 4, the vertical line designates the output of the meter which is given by K.multidot.(V.sub.R /V.sub.M), wherein K is a constant. The horizontal line designates the moisture weight (g/m.sup.2) of the paper C.sub.1 is the calibration curve when the basis weight of the paper is small. C.sub.2 is the calibration curve when the basis weight of the paper is medium. C.sub.3 is the calibration curve when the basis weight of the paper is large.
For example, when the moisture weight is 25 g/m.sup.2, the output of the meter according to calibration curve C.sub.1 is the smallest and the output of the meter according to calibration curve C.sub.3 is the largest. The reason that the output according to calibration curve C.sub.3 is larger even when the moisture weight is the same is that, when the basis weight is large, the number of times reflection and scattering occurs within the paper increases, the optical path length becomes substantially longer, the M rays are absorbed by moisture more, and the value of output V.sub.M becomes smaller.
When the moisture weight is 25 g/m.sup.2, if the output of the meter according to calibration curve C.sub.1 is regarded as the standard, there is a gap D.sub.1 between the standard and calibration curve C.sub.3 of 12.5 g/m.sup.2 when converted into moisture weight. If the gap is large, a gap d.sub.1 between an object to be measured, which is shown by a broken line, and the calibration curve C.sub.2, which is selected as the calibration curve having the characteristics nearest to that of the object, is large. This causes error of measurement. Furthermore, the output according to a conventional meter is shown as moisture weight, but generally, as a value according to which the quality of paper is to be controlled, moisture percentage is more convenient. Thus, in addition to moisture weight, the basis weight of the paper is obtained, to calculate moisture percentage.
Among the conventional meters discussed above, the FIG. 1 meter is advantageous in that it is simple and attenuation of quantity of rays is small But, since the object to be measured is only one point of the paper, there is a problem in that if the paper is thin, the miosture meter cannot be very sensitive.
In the meter of FIG. 2, since the optical axis of the light emitting portion and the optical axis of the light detecting portion are misaligned, the number of times the rays meet the paper is large. However, the range of the rays which are scattered at the paper is 180.degree. at the widest, and the rays which meet the paper only once are included. Thus, the sensitivity of the meter is not satisfactory. Also, there is a problem in that if the optical axis of the light emitting portion is shifted more from the optical axis of the light detecting portion in order to decrease the influence of rays which penetrate the paper only once, the quantity of rays is decreased.
A conventional meter is designed so that the optical axes of the light emitting portion and the light detecting portion may be shifted from each other by about 60 mm, and the distance between the upper reflector and the lower reflector may be about 6-8 mm.
In the meter of FIG. 3, since rays which do not penetrate and are not scattered by the paper many times (that is the low sensitive rays do not meet sufficiently with the water molecules) are included in the rays to be detected, there is a problem in that the sensitivity of moisture detection is low. Also, the sensitivity is different depending on whether the paper is thin or thick. Accordingly, there is a problem in that the influence of the quality of paper is great.
Moreover, in the above conventional meters, if the axis of the head containing the light emitting portion and the axis of the head containing the light detecting portion are misaligned within a horizontal plane, a large error is caused. Accordingly, there is a problem in that a mechanical or electrical correcting means is necessary for correcting the resulting error.